The invention relates to an AC-DC converter comprising
input terminals which are to be connected to the poles of a supply voltage source supplying an alternating voltage and output terminals,
rectifier means coupled to the input terminals for rectifying the alternating voltage,
an inductive element coupled to the rectifier means,
a buffer capacitance coupled to the output terminals,
a unidirectional element coupled between the inductive element and the buffer capacitance,
a switching element coupled to the inductive element for controlling a current through the inductive element,
a control circuit coupled to a control electrode of the switching element for generating a periodic control signal for rendering the switching element alternately conducting and non-conducting at a frequency f, and provided with
a first circuit part for setting a first time interval t-on during which the switching element is conducting in each period of the control signal, and
a second circuit part for setting a second time interval t-off during which the switching element is non-conducting in each period of the control signal.
Such an AC-DC converter is disclosed in U.S. Pat. No. 4,683,529. The control circuit of the known AC-DC converter renders the switching element conducting during a first time interval t-on, which is substantially constant during each half period of the alternating voltage supplied by the supply voltage source. During the first time interval t-on, the current in the inductive element increases substantially linearly. The value of t-on corresponds to the power taken at the output terminals. As the value of t-on is substantially constant during each half period of the alternating voltage, the value of the current taken from the supply voltage source, averaged over a period of the control signal, is substantially proportional to the alternating voltage. It is thus achieved that the power factor of the known AC-DC converter is comparatively high and the THD generated by the AC-DC converter is comparatively low. During the second time interval t-off, the current in the inductive element decreases substantially linearly. In the known AC-DC converter, the control circuit renders the switching element conducting again almost immediately after the current in the inductive element has become substantially equal to zero. This control of the switching element is referred to as xe2x80x9ctransition modexe2x80x9d. As the current in the inductive element is substantially zero, the same applies to the current through the unidirectional element. It is thus achieved that, when the switching element becomes conducting, only a comparatively small power dissipation occurs in the unidirectional element.
The frequency of the control signal is often chosen to be comparatively high because this enables both the inductive element and an EMI filter, which is often arranged between the input terminals and the rectifier means, to be chosen so as to be comparatively small. As a result, the AC-DC converter is comparatively small and inexpensive. However, if the power taken at the output terminals decreases, or if the amplitude of the alternating voltage supplied by the supply voltage source increases, the value of t-on is reduced by the control circuit. Also at this lower value of the power taken and/or at a higher value of the maximum amplitude of the alternating voltage, the known AC-DC converter operates in the transition mode, as a result of which the frequency of the control signal increases substantially. A drawback of the known AC-DC converter resides in that, at a high frequency, the majority of the known control circuits are insufficiently capable of sufficiently accurately controlling the time interval t-on, so that instabilities in the operation of the AC-DC converter may occur. The amount of power dissipated in the switching element also is comparatively high at a comparatively high frequency of the control signal.
It is an object of the invention to provide an AC-DC converter which can operate in a stable manner over a large range of the power taken at the output terminals and over a large range of the amplitude of the alternating voltage supplied by the supply voltage source, and which has a high power factor, a small THD and a small power dissipation in the components.
To achieve this, an AC-DC converter of the type mentioned in the opening paragraph is characterized in that the second circuit part comprises a third circuit part for setting the second time interval t-off at a value that is equal to the expression
C2*(C1*t-on+(1+C1)*t-d),
wherein t-d is a third time interval during which the unidirectional element is conducting in each period of the control signal, and C1 and C2 are parameters which have a constant value during each half period of the alternating voltage, whereby C2 greater than 0, C1= greater than 0 and t-off greater than t-d.
The third circuit part brings about that the second time interval t-off lasts longer than the time interval t-d. During the time interval xcex94t=t-offxe2x88x92t-d, the inductive element, the unidirectional element and the switching element do not carry current. Consequently, during the time interval At no power is taken from the supply voltage source, and no power is supplied to the buffer capacitance. Such a control of the switching element of an AC-DC converter is referred to as discontinuous mode. In comparison to operation in the transition mode, wherein the duration of a period of the control signal is equal to the sum of t-on and t-d, the duration of a period of the control signal is extended by At. An AC-DC converter in accordance with the invention is frequently dimensioned such that the AC-DC converter operates in the transition mode at a nominal value of the maximum amplitude of the alternating voltage and a nominal value of the power taken at the output terminals. If, for example, the power taken at the output terminals decreases or the maximum amplitude of the alternating voltage increases, then the third circuit part brings about a transition from operation in the transition mode to operation in the discontinuous mode. As a result, the increase of the frequency of the control signal is less than it would have been if the AC-DC converter had continued operating in the transition mode. By virtue thereof, the operation of the AC-DC converter remains stable and the power dissipation in the switching element is limited. It has also been found that the power factor of an AC-DC converter in accordance with the invention is relatively high over a large range of the power taken at the output terminals and over a large range of the amplitude of the alternating voltage supplied by the supply voltage source, while the THD generated by the AC-DC converter is comparatively low. In addition, the frequency of the control signal can be chosen to be comparatively high at a nominal load and a nominal value of the amplitude of the alternating voltage supplied by the supply voltage source. As indicated hereinabove, this has the advantage that the inductive element, and a possible EMI filter, can both be small.
More particularly, favorable results have been achieved with embodiments of an AC-DC converter in accordance with the invention wherein C1 greater than 0 and C2=1. In this case, the time interval xcex94t is proportional to the sum of t-on and t-d, as a result of which the ratio between the average values of the current through the inductive element in two successive periods of the control signal during operation in the discontinuous mode is equal to the ratio during operation in the transition mode. In other words, the form of the current taken from the supply voltage source remains substantially unchanged. As a result, the power factor of the AC-DC converter remains comparatively high and the generated THD remains comparatively low if the third circuit part causes the AC-DC converter to change over to operation in the discontinuous mode. Such an embodiment of an AC-DC converter in accordance with the invention can be achieved, for example, by accommodating an analog timer (formed by a first current source and a first capacitor) in the control circuit and switching on this analog timer when the switching element becomes conducting, and switching off said analog timer when the coil current becomes substantially zero. The value present in the timer (i.e. the voltage across the first capacitor) is a measure of the time interval xcex94t by which the relevant high-frequency period must be extended to obtain the desired value of t-off. The time interval xcex94t can be derived from the voltage across the first capacitor by means of a second analog timer comprising a second current source and a second capacitor. The second timer is switched on when the coil current is substantially equal to zero. The moment the voltage across the second capacitor is equal to the voltage across the first capacitor, the time interval xcex94t has elapsed and the switching element must be rendered conducting again.
In a preferred embodiment of an AC-DC converter in accordance with the invention, the constant C1 is chosen to be equal to zero and the constant C2 is chosen to be larger than 1. If the constant C1 is chosen to be equal to zero, the time interval xcex94t is not proportional to t-on+t-d, but to t-d. In practice it has been found to be comparatively easy to embody the third circuit part such that C1 is equal to zero. It has also been found that the power factor is comparatively high and the THD comparatively low over a large range of the power taken at the output terminals and over a large range of the amplitude of the alternating voltage supplied by the supply voltage source. Satisfactory results have been achieved, more particularly, using embodiments wherein, during operation, the third circuit part sets t-off at a value equal to K*(Um/(U0xe2x88x92Um)), wherein K is a constant, Um is the instantaneous amplitude of the rectified alternating voltage and U0 is the voltage between the output terminals of the AC-DC converter.
In practice it has been found that the current through the inductive element does not become exactly zero at the end of the third time interval t-d; instead an oscillation occurs whose frequency f-osc is determined by the self-inductance of the inductive element and parasitic capacitances, such as the parasitic capacitance of the switching element. As a result of this oscillation, an alternating voltage of frequency f-osc is applied across the switching element. If the switching element were to be rendered conducting while this alternating voltage has a comparatively high amplitude, then a comparatively high power dissipation would take place in the switching element. In order to preclude this, the control circuit of an AC-DC converter in accordance with the invention is preferably provided with a fourth circuit part for keeping the switching element in the non-conducting state in dependence upon the voltage across the switching element. As a result of this fourth circuit part, the switching element can only be rendered conducting when the amplitude of the alternating voltage across the switching element is comparatively low, so that the power dissipation in the switching element remains relatively small.
Immediately after the unidirectional element has stopped carrying current at the end of the time interval t-d, the unidirectional element is still comparatively highly conducting for a current in the high-resistance direction. If, when the AC-DC converter is operated in the transition mode, the switching element would be rendered conducting immediately after the unidirectional element has stopped carrying current at the end of the time interval t-d, then a comparatively high current would flow from the buffer capacitance through the unidirectional element and the switching element for a short time period. Such a current causes power dissipation, more particularly in the unidirectional element, and also reduces the xe2x80x9coverall efficiencyxe2x80x9d of the AC-DC converter. To preclude that the switching element is rendered conducting too soon after the current through the unidirectional element has become zero, the first circuit part of an AC-DC converter in accordance with the invention is preferably provided with a fifth circuit part for keeping the switching element in the non-conducting state, during a fourth time interval, after the unidirectional element has become non-conducting. Good results have been obtained with embodiments wherein the fourth time interval is inversely proportional to the maximum amplitude of the current through the inductive element during the first time interval.
In general, an AC-DC converter in accordance with the invention is designed to supply a power to a load coupled to the output terminals, which power lies within a range limited by a maximum power and a minimum power. In addition, the design often is such that the AC-DC converter operates in the transition mode at the maximum power taken. If the power taken is reduced, the frequency of the control signal increases. This increase is limited to a certain extent by the third circuit part. However, the frequency of the control signal is governed not only by the power taken but also by the amplitude of the alternating voltage supplied by the supply voltage source: the frequency of the control signal is higher at each power supplied as the amplitude of the alternating voltage is higher. If the amplitude of the alternating voltage is comparatively high and the power supplied is comparatively small, the frequency of the control signal may become unacceptably high in spite of the frequency-limiting effect of the third circuit part. To make sure that the power supplied over the same range can be set, also if the amplitude of the alternating voltage is high, it is desirable to reduce the frequency of the control signal at the maximum value of the power supplied. For this reason, it is advantageous for the control circuit of an AC-DC converter in accordance with the invention to be provided with a sixth circuit part for setting the frequency of the control signal, at a predetermined amount of power taken from the output terminals, in dependence upon the amplitude of the alternating voltage. If the amplitude of the alternating voltage is comparatively high, the sixth circuit part reduces the frequency of the control signal. As a result, the AC-DC converter operates in the discontinuous mode even if the maximum power is taken. By virtue of this comparatively low frequency of the control signal at the maximum power taken, the frequency of the control signal can no longer reach an unacceptably high value when the power taken is reduced.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.